The present invention relates to equipment installed in trunk circuits of a private branch exchanger (PBX) system for receiving call meter signals. More precisely, it relates to small sized subscriber's private metering (SPM) equipment which is operable by various types of metering pulses having a common mode or a normal mode.
There are various kinds of charging equipment installed in telephone offices for counting the calls of each subscriber for the purpose of charging a tariff. Recently the demand for private metering has increased, especially among PBX subscribers, or modern push button dialing telephone subscribers. Such equipment is called subscriber's private metering and abbreviated as SPM. Most SPM systems are controlled by their own clock and counter for operating their call meters. But some of the PBXs are provided with SPM equipment which is operated by a metering signal sent from the telephone office, and the demand for such an SPM system is increasing.
There are various kinds of metering signals used in telephone offices, but they can be classified into 50 Hz, 12 kHz and 16 kHz signals. Two modes of metering signals are used, called common mode and normal mode. The common mode signal is transmitted in each of the two-wire telephone lines having common phase (same phase) to each other and is sometimes called a parallel mode. The common mode is mainly used for the low frequency metering signal of 50 Hz. On the other hand, the normal mode signal is transmitted in the two-wire line with opposite phase to each other. Normal mode is sometimes called a differential mode signal, and is mainly used for higher frequency metering signals of 12 kHz or 16 kHz.
The SPM equipment which is operated by such metering signals, sent from the telephone office, have to change their circuit according to the type of the signal sent from the telephone office. The matching of the SPM equipment to the metering signal is done by replacing a printed circuit board, or by switching to a proper circuit among various ones which are installed in advance in the equipment and correspond to expected types of the metering signals.
In order to make clear the advantage of the present invention, prior art SPM equipment will be described briefly.
FIG. 1 is a block diagram of a prior art PBX system having an SPM function. A PBX cabinet 1 is installed in a subscriber's office, and performs as an extended branch service between a central office (CO) 9 and a branched telephone 3. Though there is shown only one telephone and one central office line 10 in the figure, usually a plurality of central office lines and telephones are interconnected and exchanged by a time division switch (TDSW) 2. Each telephone 3 is connected to the TDSW 2 via a line circuit (LN) 4 which is an interface circuit between the telephone 3 and the time division switch 2. On the central office side of the TDSW 2 is provided a central office trunk (CO trunk or COT) 5 which is an interface circuit between the central office line and the TDSW 2. This equipment is controlled by a common control unit (CC) 6. The PBX cabinet 1 is provided with an attendant console (ATT) 7, which commands the operation of the PBX manually by an operator. There may be various equipment provided in the PBX, but only the parts relevant to the present invention are shown.
Between the central office (CO) 9 and the PBX cabinet 1 is provided an SPM cabinet 8. The telephone signal and the call meter signal (sometimes abbreviated as meter signal) are sent from the central office to the PBX via a telephone line 10 which is usually a two-wire line. These signals are separated from each other in the SPM cabinet 8 and transferred to the PBX cabinet 1, respectively, through a speech line 11 and a meter line 12. In the figure, the path of the call meter signal is identified by a broken line. The call meter signal is treated by the common controller 6 and operates an indicator. The indicator may be a call meter (MET) 13, or any other display means such as a printer, or a cathode ray tube, etc.
FIG. 2 shows an example of a prior art SPM receiver for a common mode call meter signal. The equipment of FIG. 2 corresponds to the SPM cabinet 8 and COT 5 in FIG. 1. Throughout the drawings hereinafter, the same reference numerals or symbols designate the same or similar parts. The meter signal is sent from the central office together with a voice signal via a two-wire telephone line L1 and L2. The meter signal is composed of a pulse burst of 50 Hz, having the same phase to each other in both of the lines L1 and L2. The voice signal is a normal mode signal which has opposite phase to each other in these lines. The voice and meter signals are then fed to the central office trunk (COT) 5 of the PBX system.
On the central office side of the COT 5, there is provided a 50 Hz receiver 21 between the two-wire line L1, L2 and the ground, which is designed to be high impedance for voice signal frequency but becomes very low impedance for 50 Hz. So, the voice signal is fed to the COT 5 without loss, but the call meter signal of 50 Hz is shunted to the ground and does not appear in the COT 5. In FIG. 2, broken lines show the path of the call meter signal, and arrow marks show their phase indicating that they have equal phase to each other. Even if a fractional part of the 50 Hz signal appeared in the COT 5, the signal could not go further to the right in the figure. The COT 5 is provided with a hybrid circuit 23 which does not convert the two-wire common mode signal to a 4-wire signal, wherein the voice signal is converted to the 4-wire signal because of having a normal mode. In the figure, a symbol BN designates a balancing network for the hybrid circuit 23. The output of the hybrid circuit 23 is fed to the TDSW 2, and then switched to the destination of telephone equipment 3 (not shown).
The above is a description of the flow of the signal from the central office to the destination telephone equipment. The flow of the signal from the telephone equipment 3 to the central office is also transmitted through the same path, but in a reverse direction to that described above.
A reference numeral 24 designates an interface circuit which terminates the two-wire lines L1 and L2, and provides various functions such as dial pulsing, supervising, ringing and so on. These functions are controlled by a controller 25, but such operations are all the same as with ordinary PBX systems, and they are not particularly relevant to the present invention, therefore, further description is omitted for the sake of simplicity.
When the call meter signal is sent from the central office, the signal runs through the 50 Hz receiver 21 to the ground, therefore the call meter signal is detected by the 50 Hz receiver 21. Then, the 50 Hz receiver 21 closes a switch 26 to make a loop between the controller 25 and the lines MT, MR. This closed loop is detected by the controller 25 and a signal indicating that a meter signal is received is sent to the common controller (CC) 6.
FIG. 3 is an example of a prior art SPM receiver for a call meter signal of normal mode. In the figure, a broken line indicates the flow of the call meter signal, and arrow marks indicate that the phase of the meter signal is opposite to each other in the lines L1 and L2. Such a circuit is used for call meter signals of 12 kHz or 16 kHz. On the lines L1 and L2 is provided a band rejection filter 31 which is tuned sharply to 12 kHz or 16 kHz, corresponding to the frequency of the call meter signal. So, the voice signal (400-3,600 Hz) is transmitted to the interface circuit 24 without loss. While the call meter signal is branched to a 12 kHz/16 kHz receiver 32 by a transformer T 33. The circuit is designed such that the input impedance of the transformer 33 is very high for voice frequency, but also designed to become low impedance for 12 kHz/16 kHz. Thus, the call meter signal sent from the central office is detected by the 12 kHz/16 kHz receiver 32. The operation of the remaining parts of the PBX is the same as that of the circuit of FIG. 2.
Additional prior art SPM equipment is disclosed in, for example,
Japanese Laid Open Patent 58-19071 (published on Feb. 3, 1983) by K. Sato, or PA1 Japanese Laid Open Patent 58-159054 (published on Sept. 21, 1983) by K. Sato
As has been described above, the prior art SPM equipment has to change their circuits corresponding to the call meter signals sent from the central office. The matching of the circuit to the meter signal is done by changing a printed circuit board. Some of the universal SPM equipment is installed with various circuit boards corresponding to expected types of the meter signals to deal and the circuit is switched manually to a proper one according to the type of signal sent from the central office. Therefore, the SPM cabinet becomes large, and expensive.